Low Cost Corrosion Damage Mitigation And Improved Fatigue Performance Of Low Plasticity Burnished 7075-T6

ABSTRACT

Low plasticity burnishing (LPB) has been investigated as a surface enhancement process and corrosion mitigation method for aging aircraft structural applications. Compressive residual stresses reaching the alloy yield strength and extending to a depth of 1.25 mm (0.050 in.), deeper than typical corrosion damage, is achievable. Excellent surface finish can be achieved with no detectable metallurgical damage to surface and subsurface material.

Salt fog exposures of 100 and 500 hrs. reduced the fatigue strength at 2×10 6 cycles by fifty-percent. LPB of the corroded surface, without removal of the corrosion product or pitted material, restored the 2×10 6 fatigue strength to greater than that of the original machined surface. The fatigue strength of the corroded material in the finite life regime (10 4 to 10 6 cycles) after LPB was 140 MPa (20 ksi) higher than the original uncorroded alloy, and increased the life by an order of magnitude.

Ease of adaptation to CNC machine tools allows LPB processing at costs and speeds comparable to machining operations. LPB offers a promising new technology for mitigation of corrosion damage and improved fatigue life of aircraft structural components with significant cost and time savings over current practices.

INTRODUCTION

High cycle fatigue damage, often initiated from corrosion pitting, is a growing threat to safety and performance of Naval aircraft. Maintenance requirements to inspect for fatigue damage, replace parts, and rework to remove corrosion damage increase the cost of operation. The down time required to perform inspections and repairs, during which aircraft are not available for combat, significantly impacts military readiness. Estimated annual costs for corrosion inspection and repair of Naval aircraft alone exceed one billion dollars. Currently, more than 30% of military aircraft are over 20 years old and over 90% are expected to exceed a 20 year life by the year 2015.1

Corrosion pits are a common site of fatigue crack initiation in the aluminum alloy structural components of Naval aircraft. Corrosion pitting results in intergranular corrosion to a depth depending upon the time of exposure, temperature and the service environment of the aircraft. The pronounced fatigue strength reduction caused by salt pit corrosion is well established for both steels2 and aluminum alloys3 and results typically in the reduction of the endurance limit to nominally half of the uncorroded value. Common overhaul practice requires hand re-work or machining to remove the pitted layer followed by shot peening as a surface enhancement technique to improve fatigue life, a time consuming and costly practice.

New surface enhancement technologies have been developed which can provide a layer of compressive residual stress of sufficient depth to effectively eliminate the influence of the salt pit corrosion. Laser shock peening (LSP)4,5 has been demonstrated to produce a pronounced increase in fatigue life of samples containing deep FOD. Unfortunately, laser shocking is extremely expensive to perform, and slow and difficult to incorporate into aircraft manufacturing and overhaul shop environments. More recently, low plasticity burnishing (LPB)6 has been demonstrated to provide comparable depth and magnitude of compressive residual stress at far lower cost than for LSP. The LPB process can be performed on conventional CNC machine tools at costs and speeds comparable to conventional machining operations such as surface milling. The residual stress distributions developed in IN7186 and in Ti-6Al-4V7 produced by LPB have exceeded 1 mm in depth, well beyond the depth of typical corrosion pits which serve as the initiation sites for fatigue cracks.

Recent work in modeling8 of corrosion pit crack growth in 7075-T6 has indicated that the pits can be treated as semi-elliptical cracks having depths on the order of average pit depth for the purpose of predicting the degradation of fatigue strength caused by corrosion. Therefore, if a layer of compressive residual stress of sufficient magnitude and depth can be induced into the surface of the part, to a depth greater than the existing pits or micro-cracks, the growth of fatigue cracks might be arrested.

The purpose of this study was to investigate the effectiveness of LPB in creating a layer of compressive residual stress in 7075-T6 aluminum. The effect of the compressive layer on the fatigue strength of salt fog-corroded 7075-T6 machined surfaces was used to test performance.

EXPERIMENTAL TECHNIQUE

Material

Aluminum alloy 7075-T6 was acquired in the form of ½ inch plate to AMS 4045. In the -T6 heat-treated condition, the material was found to have a hardness of 89 HRB and electrical conductivity of 33.0% IACS. Chemistry was verified to be within limits of the AMS 4045 specification as shown in Table 1.

Tensile properties were verified as UTS=601 MPa (87.3 ksi), 0.2% yield strength of 542 MPa (78.7 ksi) with an elongation of 11%.

Low Plasticity Burnishing

All currently available methods of surface enhancement develop a layer of compressive residual stress following mechanical deformation. The methods differ primarily in how the surface is deformed and in the magnitude and form of the resulting residual stress and cold work (plastic deformation) distributions developed in the surface layers.

 

TABLE 1
7075-T6 ½ in. Plate Composition

 

Conventional air-blast shot peening is routinely applied to a wide variety of aircraft components. High velocity impact of each particle of shot produces a dimple with a region of compression in the center. Typical compressive residual stress distributions reach a maximum approaching the alloy yield strength, and extend to a depth of 0.05 to 0.5 mm (0.002 to 0.020 in.) The magnitude of compression achieved depends primarily upon the mechanical properties of the alloy. The depth of the compressive layer and the degree of cold working depend upon the peening parameters including shot size, velocity, coverage and impingement angle. Because each shot impacts the surface at a random location, peening for sufficient time to achieve uniform surface coverage results in many multiple impacts producing a highly cold worked surface layer.9

Conventional shot peening produces from 10% to 50% cold work, much more than from grinding, machining, or other common surface finishing processes.10 Cold work is cumulative, and repeated applications of shot peening can produce even more than 50% cold work. Both the depth and degree of cold working increase with peening intensity, with the most severe cold working at the surface. Surface compression often decreases during shot peening of work hardening materials as the yield strength of the surface increases with continued cold working.

The concept of low plasticity burnishing (LPB) originated as a means of producing a layer of compressive residual stress of high magnitude and depth with minimal cold work.11 The process is characterized by a single pass of a smooth free rolling spherical ball under a normal force sufficient to plastically deform the surface of the material, thereby creating a compressive layer of residual stress. The process is shown schematically in Figure 1. The ball is supported in a fluid bearing with sufficient pressure to lift the ball off the surface of the retaining spherical socket. The ball is in mechanical contact only with the surface to be burnished and is free to roll on the surface of the work piece.

 

Figure 1 – Low Plasticity Burnishing schematic.

 

Although the tool designs and hydraulic systems differ, the LPB tooling is similar to “deep rolling” tools using a hydrostatically supported burnishing ball.12,13,14 The LPB and deep rolling processes differ in the method of use and the level of cold work generated in developing the compressive layer. X-ray diffraction peak broadening and micro-hardness distributions generated by shot peening and deep rolling reveal that deep rolling produces cold work greater than shot peening. In contrast, LPB typically produces cold work an order of magnitude lower than shot peening.

Using CNC positioning, the tool path is controlled so that the surface is covered with a series of passes at a separation maintained to achieve maximum compression with minimum cold working. The tool may be moved in any direction along the surface of a complex work piece, as in a typical multi-axis CNC machining operation.

The burnishing ball develops subsurface Hertzian contact stresses in the work piece. These stresses act parallel to the plane of the surface and reach a maximum beneath the surface. With sufficient pressure applied normal to the surface, the subsurface stress exceeds the yield strength of the work piece material, thereby producing deep subsurface compression. The normal force required and the depth at which yielding first occurs depend upon the ball diameter.

The speed of burnishing up to 500 sfm has been found to have no effect upon the residual stress distribution produced. This allows application of the process at the highest practical CNC machining speeds.

The surface residual stress depends upon the normal force, feed and mechanical properties of both the ball and work piece. Lateral plastic deformation of the surface is necessary to achieve surface compression.

Processing parameters have been established empirically. With a poor choice of processing parameters, the surface can be left nearly stress free or even in tension. Empirical optimization has been used successfully to select parameters that leave the surface in compression.

 

Figure 2 – LPB tool positioned for burnishing a coupon in a 20 HP vertical CNC mill.

 

The LPB tool designed to fit a CAT-40 tool holder in a Haas HP vertical CNC mill is shown in Figure 2. The quill of the machine is not rotated. The swivel links in the hydraulic hose allow exchange of the tool to and from the tool holder so that LPB processing can be incorporated into standard machining sequences in existing CNC machine tools. Injection of the fluid through the quill of the mill is also possible in a suitably equipped machine. With minor modification, the apparatus can be adapted to most horizontal and multi-axis mills, or lathes.

The control apparatus for the hydraulic system provides a constant flow of fluid to support the burnishing ball and a computer controlled feedback system to maintain the desired normal force and fluid pressure. The burnishing force and tool feed can be varied in order to “feather” the residual stress field, thereby providing a smooth transition at the perimeter of the burnished zone or to produce a distribution of residual stress appropriate for a specific application or applied stress field.

The burnishing ball is the only wear prone component of the LPB tooling. High chromium steel, beta-silicon nitride, and sintered tungsten carbide balls, readily available from ball bearing applications, have been used successfully in the current apparatus. The surface finish achievable depends upon the finish of the ball. Bearing balls are commonly available with finishes of grade 25 (25 micro-inch), or better at costs less than cutting tool inserts.

X-ray Diffraction Characterization

Diffraction peak broadening, measured along with the residual stress, allows the amount of damage developed by surface enhancement methods to be accurately assessed. The method of quantifying the degree of cold working of metals, by relating the x-ray diffraction peak broadening to the equivalent true plastic strain, has been described previously.10 The distribution of cold work as a function of depth into the deformed surface can be expressed in terms of the equivalent true plastic strain. If the degree of cold work is taken to be the equivalent amount of true plastic strain, the degree of cold work is then cumulative and is independent of the mode of deformation. Thus, the subsurface yield strength distribution can then be estimated from true stressstrain curves.10 The macroscopic residual stress, of primary interest in design and life prediction, is determined in the conventional manner from the shift in the diffraction peak position.15,16,17

High Cycle Fatigue Testing

Four-point bending was the HCF testing mode selected to provide maximum sensitivity to the surface condition.18 Fatigue testing was conducted at room temperature on a Sonntag SF-1U fatigue machine under constant sinusoidal load amplitude at 30 Hz, R=0.1.

A bending fatigue specimen having a trapezoidal cross section was designed especially for the testing of highly compressive surface conditions created by surface enhancement methods. The test specimen provides a nominally 0.5 in. wide by 1-in. long region under uniform applied stress to minimize scatter in fatigue testing. The original gage section thickness of nominally 0.375 in. was chosen to be adequate to support the tensile stresses induced in the back of the specimen when a deep highly compressive layer was formed on the test surface. The gage section thickness was then reduced to 0.25 in by milling the backside to insure failure out of the highly compressive surface in four point bending. The HCF samples were finished machined by milling using conventional end milling to simulate the surface conditions including residual stress and cold work that would be present on a machined structural aircraft component manufactured from 7075-T6.

Base line S/N curves were developed for the asmachined condition and the machined condition plus LPB processing. S/N curves were then developed for specimens that had been machined and then exposed to either 100 or 500 hours in the salt fog environment. Half of the specimens given the 100 and 500-hour exposures were then LPB processed. S/N curves were then generated for the as-corroded and corroded plus LPB specimen groups.

Salt Fog Corrosion Exposure

The salt fog corrosion exposure was performed at 35° C per ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus. The fog produced was such that 1.0-2.0 ml/hr of 5 ± 1 mass percent NaCl aqueous solution collected on each 80 cm2 horizontal surface. The pH of the solution was maintained between 6.5 and 7.2. The salt fog exposure was performed at the Naval Air Depot at Cherry Point using a model TTC600 chamber manufactured by Q-Fog Corporation.

The specimens were exposed in two groups with the test surface horizontal for 100 and 500 hours. Following exposure to the salt fog, the samples were soaked and then rinsed in tap water, followed with a distilled water rinse to remove any salt solution remaining, and then dried. Patches of gray and white corrosion product evident on the surface of the samples were identified by x-ray diffraction as α-Al2O3. The corrosion product was not removed prior to testing or LPB processing.

RESULTS AND DISCUSSION

Residual Stress Distributions

The residual stress distributions developed by the LPB parameters used in this investigation are shown in comparison to conventional shot peening in Figure 3. Shot peening to an 8A intensity with CW14 shot for 200% coverage, a typical practice, produces compression to depths on the order of 0.2 mm (0.008 in.) reaching a maximum on the surface of nominally 350 MPa (50 ksi). The repeated impacts of the shot necessary to achieve coverage resulted in approximately 45% cold work at the shot peened surface. In contrast, the LPB parameters used in this investigation produced maximum compression below the surface on the order of -650 MPa (-94 ksi). Maximum cold work of nominally 20% occurs below the surface and the compressive layer extends more than one millimeter (0.040 inches) into the surface. LPB produced both a higher magnitude of compression and a greater depth than shot peening, well beyond the depth of corrosion pitting.

 

Figure 3 – Comparison of residual stress and half width distribution in shot peened and LPB processed 7075-T6 Aluminum.

 

No attempt was made to optimize the LPB parameters used in this investigation other than to produce a deep layer of compression. Previous experience with LPB of IN718, Ti-6Al-4V, and 4340 steel indicates that with proper selection of burnishing parameters, the amount of cold working could be reduced while maintaining comparable depth of compression. These parameters, including normal force, ball size, ball material, and feed, could easily be adjusted if further reduction of cold working is found to improve resistance to corrosion.

The effect of LPB processing on the surface finish of the original machined material and after 100 and 500 hrs. exposure to salt fog are shown in Table II. These are the results of three repeat measurements made on each of two samples representing the conditions shown. The effect on the original machined surface was to reduce the surface roughness from nominally 40 to 8 μin, Ra. Salt fog exposure produced a very rough and non-uniform surface finish resulting in a wide spread in roughness in the corroded condition as indicated by the high standard deviations for the corroded specimens. For either 100 or 500 hr. exposure, the LPB reduced the surface roughness by nominally 100 μin, Ra.

Table II

Corrosion Damage

Fractographic examination of the fatigue failures and macroscopic examination of the exposed surfaces revealed that salt fog exposure resulted in uniform corrosion of the test surfaces. Pit depths averaged 0.004 – 0.005 in. with some pits extending to 0.01 in. After penetration of the surface, the corrosion crevices often were observed to progress laterally, thereby delaminating layers of material apparently following the elongated grain boundaries produced by rolling of the plate. Although no significant difference was observed in the type or depth of corrosion pits produced by the 100 and 500 hr. exposure, a greater density of pitting was evident for the 500 hr. exposures.

 

Figure 4 – Macroscopic appearance of a specimen after 100-hr. exposure showing patches of Al2O3 corrosion product.

 

A typical macroscopic view of a corroded specimen surface after 100 hrs. exposure is shown in Figure 4. The macroscopic view of a typical corrosion pit crack nucleation site on a machined +100 hr. salt fog exposed specimen is shown in Figure 5A. The same corrosion pit nucleation site is shown in the SEM micrograph in Figure 5B.

 

Figure 5A- Arrow indicates macroscopic appearance of fatigue origin at the site of corrosion after 100 hr. salt fog exposure.

Figure 5B – Microscopic appearance of the fatigue origin at the site of corrosion pit shown in 5A after 100 hr. salt for exposure.

HCF Performance

The S/N curves generated for the as-machined, machined + LPB, corroded, and corroded + LPB specimens are summarized in Figure 6. Fatigue tests were terminated after nominally 2×106 with a few exceptions running out to as much as 107 cycles. Most of the conditions for which the samples were tested appeared to exhibit an endurance limit, implying infinite life below some threshold stress level. Because of the limited testing time and small number of tests performed, however, this cannot be confirmed, especially for the longer running tests after LPB processing. Therefore, the results are considered and compared in terms of the fatigue life at nominally 2×10 6 cycles. The data are presented as semi-logarithmic S/N curves at R=0.1 in terms of the maximum stress.

The machined (end milled) surface condition produced an apparent endurance limit behavior and a fatigue strength at nominally 2×10 6 cycles on the order of 200 MPa (30 ksi). Salt fog exposure for either 100 or 500 hrs. reduced the fatigue strength in the finite life range, from 10 4 to 2×10 5 cycles, slightly, perhaps on the order of 35Mpa (5 ksi), but reduced the fatigue strength at 2×10 6 to nominally half that of the original machined surface before corrosion. Loss of nominally half the extended life fatigue strength following salt fog corrosion appears to be typical of the degradation reported in the literature. 3

LPB of the machined surface without corrosion exposure produced the highest fatigue strength at any life. Burnishing directly over the corroded surface produced by either 100 or 500 hr. salt fog exposure resulted in nearly identical improved fatigue performance, even exceeding that of the original machined surface. LPB appears to have at least fully restored the extended life fatigue strength at 2×10 6 to that of the original as machined surface. Several data points indicate improved long-term fatigue strength by as much as nominally 35 MPa (5 ksi) at lives exceeding 2×10 6 . In the finite life regime between 10 4 and 10 6 cycles, LPB processing of either the original machined or the machined + 100 or 500 hr. salt fog corrosion has increased the life at a given stress level by a factor of 10. Considered in terms of the increased fatigue strength at a fixed life, the fatigue strength at 10 5 cycles after LPB processing of the machined or corroded surfaces improved by nominally 170 MPa (25 ksi), a pronounced effect on a material having high cycle fatigue strength on the order of 205 MPa (30 ksi).

Fractography

Fatigue failures occurred in the as-machined specimens exclusively from the end milled-end cut machined surfaces. Generally, single fatigue origins occurred in specimens tested at lower stresses with a normal tendency toward multiple origins in specimens tested at higher stress levels. Specimens that were machined and then subjected to 100 or 500 hr. salt fog exposures suffered pitting which degraded fatigue strength relative to the as machined condition.

 

The fatigue strength degradation was the same for both exposure times despite a greater population of pits in the 500 hr. exposed specimens.

The typical macroscopic appearance of a 100-hr. exposure is show in Figure 5A. Fatigue failures in the salt fog exposed specimens initiated exclusively from corrosion pits. Specimens tested at lower stresses generally exhibited origins from a single pit, while specimens tested at higher stress levels tended to have multiple nucleation sites from separate pits. Pit depths were generally in the range of 0.004 – 0.005 in., although a few as deep as 0.010-in. were observed. Figure 5B shows a SEM micrograph of the same corrosion pit shown in 5A.

The effect of LPB after machining (no corrosion) was to drive fatigue origin subsurface by as much as 0.04 – 0.05 in. This is evidently beneath the compressive layer produced by LPB. Figure 7A shows the macroscopic appearance of the subsurface origin indicated by the arrow. A SEM micrograph in Figure 7B shows the same origin at higher magnification. Arrows in 7B indicate the fatigue crack growth emanating in different directions from the subsurface origin.

 

 

LPB treatment after salt fog exposure restored the fatigue strength too greater than as-machined levels. The strength restoration after LPB was the same for both the 100 and 500 hr. exposed conditions. Even after LPB processing, the corroded surface with pits was still very much in evidence, although the surface finish was substantially improved. The effect of LPB on fatigue failures in the corroded specimens was to drive the fatigue origin subsurface by 0.04 – 0.05-in. in the same manner as for the LPB processing of asmachined, non-corroded specimens. Figure 8A shows the macroscopic aspect of a subsurface origin (arrow) after 100 hr. salt fog exposure + LPB. The SEM micrograph, Figure 8B, shows the same origin area. Here, with local crack growth directions indicated by black arrows, it can be seen that there are actually two subsurface origins. Moreover, the nearby surface pit indicated by the while arrow was rendered innocuous by the highly compressive stress produced by LPB, and did not serve as a fatigue initiation site.

 

CONCLUSIONS

Salt fog exposures of 100 and 500 hrs. produced corrosion pits on the order of 0.005 in. with some extending to 0.010 in. As has been widely reported previously, fatigue cracks initiating from the salt pits reduced the 2×106 fatigue strength to half its initial value.

LPB applied over the corroded surface without removing either the corrosion product or the pitted alloy layer resulted in full restoration of the fatigue strength at 2×106 cycles for either exposure. The surface finish was markedly improved. Fatigue crack nucleation from corrosion pits was eliminated. Failure after LPB processing occurred from subsurface nucleation sites in all cases. The fatigue life, at any stress examined above the endurance limit, was improved by nominally an order of magnitude. Fatigue life improvement from LPB processing is attributed to the introduction of a layer of compressive residual stress of sufficient depth and magnitude to effectively close cracks emanating from corrosion pits shallower than the layer of compression, rendering them innocuous and altering the mode of fatigue crack nucleation. The compressive layer then retards the growth of fatigue cracks, which eventually nucleate, producing an order of magnitude life increase at stresses above the endurance limit. LPB has been demonstrated to restore the fatigue performance of severely salt fog corroded aluminum alloy 7075-T6. The ease of implementation in standard CNC machine centers offers the possibility of employing LPB as a low cost effective means of mitigating corrosion fatigue failures in aluminum structural aircraft components.

REFERENCES

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